Moving forwards: noninvasive imaging of the eye

March 1, 2013

US pachymetry offers no advantage over optical noncontact methods, according to Dr Savini

"I believe that ultrasound (US) pachymetry does not offer any advantage over optical noncontact methods such as Scheimpflug imaging," stressed Dr Giacomo Savini (GB Bietti Eye Foundation IRCCS, Rome, Italy) when discussing a recent investigation on the precision of a new Scheimpflug and Placidodisk analyser.1 "My group has investigated all currently available Scheimpflug cameras over the last five years. Our aim has been to understand what do these instruments really measure and calculate and how can the provided values be applied in the clinical setting."

Recently, Dr Savini revealed that he had started collaborating with the School of Optometry and Ophthalmology and Eye Hospital of the Wenzhou Medical College (Wenzhou, Zhejiang, China), where a group of researchers have similar interests and founded the Institute for Assessment and Application of Ophthalmic and Optometric Instruments (IAAO).

"Together we planned to evaluate the features of several ophthalmic devices, one of which was the Sirius (CSO, Florence, Italy), a Scheimpflug camera combined with a Placido disk topographer," he continued. "Given the utmost importance of thinnest corneal thickness (TCT) measurements in patients undergoing refractive surgery, we wanted to test the reliability of the Sirius TCT measurements and assess agreement with ultrasound pachymetry, which may still be considered as the gold standard."

To assess the precision of this new instrument the team employed the same methodology as previous studies, according to the definitions adopted by the International Organization for Standardization. Healthy subjects were examined with the ScheimpflugPlacido topographer and the central corneal thickness (CCT) and thinnest corneal thickness (TCT) were recorded after three consecutive measurements.

"We had to repeat measurements three times with one operator to calculate the intraoperator repeatability and three more times within one week to calculate the intersession reproducibility," said Dr Savini. "In the second part of the study we compared the Scheimpflug measurements to those achieved by standard ultrasound pachymetry. So, the protocol was not complicated and we had no difficulties."

High repeatability

The team found that there was high intrasession and intersession repeatability for the ScheimpflugPlacido measurements. "Achieving the same value in different measurements taken on the same day or in different sessions confirms the high precision of this rotating Scheimpflug camera combined with a Placido disk topographer," said Dr Savini. "This finding makes me confident on the measured values of TCT when screening candidates for refractive surgery and increases the safety of both LASIK and PRK."

Additionally, there was a high level of agreement between the combined Scheimpflug-Placido instrument and US pachymetry measurements. Dr Savini continued, "Actually, I did not expect such a good level of agreement between Scheimpflug and ultrasound measurements. This finding makes me even more confident about the fact that I routinely use Scheimpflug measurements to assess preoperative corneal thickness in patients undergoing LASIK and PRK."